too much of a good thing
US /tuː mʌtʃ əv ə ɡʊd θɪŋ/
UK /tuː mʌtʃ əv ə ɡʊd θɪŋ/
1.
something that is enjoyable or beneficial but becomes harmful or unpleasant if there is an excessive amount of it
:
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Eating chocolate is nice, but too much of a good thing can make you sick.
•
Even a vacation can be too much of a good thing if it lasts for months.